An HEV is a vehicle that has a propulsion system that consists of at least one electric motor or electric machine in combination with at least one other power source. Typically, the other power source is a gasoline or diesel engine. There are various types of HEVs depending on how the electric motor(s) and other power source(s) are combined with one another in order to provide propulsion for the vehicle, including series, parallel and compound HEVs.
Various hybrid powertrain architectures are known for managing the input and output torques of various propulsion systems in HEVs, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to an energy storage system, comprising a battery pack. The internal combustion engine in a series HEV is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack.
Parallel HEV architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel HEV powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions (i.e. input split, output split and compound split configurations) thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and two-mode operation.
Such complex EVT HEVs utilize one or more electric machines and require advanced energy storage systems (ESS) to supply electrical energy to and receive and store electrical energy from these machines. The ESS typically incorporates a battery pack and associated monitoring and control electronics and algorithms. Given the dynamics associated with operation of an HEV, particularly the constant flow of power into and out of the ESS, the ESS plays a critical role in the operation of these vehicles. The critical role of the ESS in these vehicles imposes a number of requirements on ESS performance, including both operational and service life requirements.
Significant attention has been given to maintaining the operational performance of batteries used as the ESS in HEV applications. Particular attention has been given to various aspects of maintaining the battery pack state of charge (SOC). The SOC is defined generally as the ratio of the residual charge in a battery relative to its full charge capacity. Various hardware and software control strategies have been adjusted for determining and maintaining the SOC of the battery.
While understanding and maintaining the SOC of the battery is critical to its performance in HEV applications, it is not the only important characteristic of the battery. Another critical characteristic of batteries used in HEV applications is the useful life of the battery or battery pack. For example, it is known that secondary batteries, such as Ni-MH batteries, have limited amp-hour throughput that defines their useful service life. The amp-hour throughput or capacity of the battery is the integral of the absolute value of the power flowing through the battery as a function of time as it is constantly charged and discharged in service.
Therefore, since the battery has a limited life, in order for HEVs to compete with other propulsion technologies, it is desirable in some applications to utilize control strategies that will permit the service life of the battery to be managed to particular levels based upon the various parameters that effect battery life such as, amp-hour throughput, overvoltage/undervoltage, temperature.